1. Field of the Invention
The invention relates generally to the field of seismic evaluation of formations in the Earth's subsurface. More specifically, the invention relates to methods for determining seismic attributes in specific formations in the subsurface to a relatively high lateral and vertical resolution.
2. Background Art
Seismic surveying techniques are known in the art for determining structure and composition of rock formations in the Earth's subsurface. Reflection seismic techniques known in the art include deploying an array of seismic sensors above a part of the subsurface to be evaluated. The sensor array may be deployed on the land surface, or in marine surveys may be towed by a vessel near the surface of a body of water or may be deployed in a selected pattern on the water bottom. The array is typically rectilinear in shape and has substantially uniform spacing between individual sensors in the array. A seismic energy source is deployed near the array of seismic sensors and is actuated at selected times. Signals are detected by the sensors in the array and are recorded. The recordings are typically indexed with respect to the actuation time of the seismic energy source. The seismic signals typically include events caused by seismic energy reflecting from acoustic impedance boundaries in the subsurface. The time indexed records from each sensor are then processed to generate images of the rock formations, and to evaluate their petrophysical properties such as fluid content, mineral composition and fractional volume of pore space (“porosity”).
Seismic surveying known in the art has relatively limited vertical and lateral resolution. Resolution limitations result from the fact that the Earth's subsurface functions as a low pass filter to seismic energy. Typically only relatively low frequency seismic energy is able to travel from the source, through the subsurface and back to the seismic sensors having retained enough energy to be detected above the noise. The frequencies are usually below 80 Hz. Images of the subsurface can be formed by summing or “stacking” of the recorded signals in various manners and by a process known as migration. The vertical resolution obtained is determined by the dominant frequency associated with penetration to the depth in the subsurface of the rock formations being analyzed. The lateral resolution obtained depends on the aperture size and sensor spacing used for the signal collection of that portion of the data that is migrated. Various schemes are employed for the extraction of velocity versus depth and for geological interpretation. One example of a migration technique is described in U.S. Pat. No. 6,466,873 issued to Ren et al.
When seismic surveys are conducted with large arrays of sensors deployed on the seabed, for example, the lateral resolution of the images formed by migration techniques will again be determined by the selected sensor array aperture size and sensor spacing. The array aperture and sensor spacing ultimately will be limited by the cost in data processing time. Each point in the resulting images results from an aperture that has been moved along a much larger array of sensors and thus the image point is a specular point. Such fixed arrays are used for various reservoir studies.
It is known in the art to perform seismic surveys repeatedly over a same area of the Earth's subsurface in order to determine changes in spatial distribution of fluids in the subsurface formations. Changes in spatial distribution of fluid over time can result from extraction of fluids, for example, producing hydrocarbon from the formations. It is desirable to have a method for seismic surveying that provides increased resolution as compared with techniques known in the art, for among other purposes, to be able to determine more precisely changes in spatial distribution of fluids disposed in subsurface rock formation as fluids are extracted from such formations.